Here is a list of commands to "
Edit Cell Path Grid" from the TES CS wiki.
Basically, you have to find the button at the top of the CS window (same area as light on/off, landscape editing) and turn pathgrid editing on. Then use the commands in the link above (right-click to add a new node, ctrl+right-click to add a new node linked to the currently selected node, left-click to select, ctrl+left-click to link/unlink selected to an existing node)
Using those, just create suggested walking paths for AI actors. They'll move from point to point using any connected path that seems appropriate to them. If you create a zig-zag as the only path, they'll walk in a zig-zag pattern. (unless they're attacking something, in which case they often ignore the grid) If you create an 'X-inside-a-box' pattern, they'll choose a more or less straight path from where they are to where they want to go, using the available options.
If you want more natural walking patterns, you'll have to consider each potential path, rather than using the X-in-a-box method. It might end up looking more like a spider web. Just don't go overboard with points, the simpler the better in most cases.
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